Lightweight multiwall sheet with high stiffness and method of making it

ABSTRACT

A multiwall sheet ( 30 ) comprising a first wall ( 2 ); a second wall ( 4 ); a plurality of ribs ( 8 ) extended between the first and second walls ( 2, 4 ); and a plurality of mid-ribs ( 6 ), wherein one mid-rib ( 6 ) extends between each two adjacent ribs ( 8 ), wherein the mid-ribs ( 6 ) extend at a mid-rib angle ( 18 ) relative to an adjoining rib ( 8 ), and wherein the mid-rib angle ( 18 ) is not 90° relative to a rib ( 8 ) to which it attaches, wherein the mid-ribs ( 6 ) attach only to adjacent ribs ( 8 ), and wherein the mid-ribs ( 6 ) do not attach to either the first or second walls ( 2, 4 ).

BACKGROUND

In the construction of naturally lit structures (e.g., greenhouses, pool enclosures, solar roof collectors, conservatories, stadiums, sunrooms, industrial buildings, residential buildings and so forth), glass can be employed as a transparent structural element, e.g., windows, facings, and roofs. However, polymer sheeting can replace glass in many applications due to several notable benefits.

Glass panel roofing systems generally provide good light transmission and versatility. However, the initial and subsequent costs associated with these systems can limit their application and overall market acceptance. The initial expenses associated with glass panel roofing systems can include the cost of the glass panels themselves as well as the cost of the structure, or structural reinforcements, that are employed to support their weight. After these initial expenses, operating costs associated with the inherently poor insulating ability of the glass panels can result in higher heating expenses for the owner. Yet further, glass panels are susceptible to damage caused by impact or shifts in the support structure (e.g., settling), which can result in high maintenance costs.

SUMMARY

Disclosed herein are multiwall sheets, such as high stiffness and lightweight multiwall sheets, articles, and methods of making the same.

In an embodiment, a multiwall sheet comprises a first wall; a second wall; a plurality of ribs extended between the first and second walls; and a plurality of mid-ribs, wherein one mid-rib extends between each two adjacent ribs, wherein the mid-ribs extend at a mid-rib angle relative to an adjoining rib, and wherein the mid-rib angle is not 90° relative to a rib to which it attaches, and wherein the mid-ribs attach only to adjacent ribs.

The above described and other features are exemplified by the following figures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are exemplary embodiments, not necessarily drawn to scale, are meant to be illustrative and not limiting, and wherein the like elements are numbered alike.

FIG. 1 is a partial, cross-sectional view of a multiwall sheet with flat mid-rib.

FIG. 2 is a partial, cross-sectional view of a multiwall sheet with repeatedly inclined mid-rib.

FIG. 3 is a partial, cross-sectional view of multiwall sheet with alternating inclined and declined mid-rib.

FIG. 4 is a graphical representation of the load versus deflection characteristics of a multiwall sheet of FIG. 1 as compared to a multiwall sheet of FIG. 2.

FIG. 5 is a graphical representation of the deflection versus weight for multiwall sheets of FIG. 1 having a variety of wall and rib thicknesses (resulting in different weights) as compared to multiwall sheets of FIG. 2 and FIG. 3.

FIGS. 6-9 are illustration partial, cross-sectional views of multiwall sheets having different mid-rib configurations.

FIG. 10 is a graphical representation of the wind pressure versus deflection performance for multiwall sheets having the mid-rib configurations shown in FIG. 6.

FIG. 11 is an illustration of a multiwall sheet roofing construction.

FIG. 12 is an illustration of a cross-sectional view of a multiwall sheet having two edges engaged.

FIG. 13 is an illustration of a multiwall sheet.

DETAILED DESCRIPTION

Multiwall polymeric panels have been produced that can exhibit improved impact resistance, ductility, insulative properties, and can have lower weight than comparatively sized glass panels. As a result, these characteristics can reduce operational and maintenance expenses. Polymer sheeting can reduce breakage and maintenance costs in applications wherein occasional breakage caused by vandalism, hail, contraction/expansion, and the like, is encountered. Polymer sheeting can have reduced weight compared to glass. This can make polymer sheeting easier to install in comparison to glass and can reduce the load-bearing requirements of the structure on which they are installed. Additionally, polymer sheeting can provide improved insulative properties compared to glass which can reduce heating and/or cooling costs and positively impact overall market acceptance of polymer sheeting.

Low weight multiwall sheets that possess high stiffness to weight ratio without significantly affecting other performance characteristics (optical, thermal, acoustic, and the like) are desired in the industry.

Multiwall sheets can be efficient structures for use in roof and wall panels. Various arrangements of multiwall sheet internal components to improve the performance have been previously attempted. Lightweight multiwall sheets can reduce manufacturing and installation cost, yet to effectively resist service loads without requiring additional supporting structure, these sheets can depend on high stiffness. The lightest conceived multiwall sheet consists of three walls with ribs there between to connect the walls into a single sheet. In such a three wall configuration weight reduction with improved stiffness can be challenging due to limited degrees of freedom in the structural design.

Examples of known multiwall sheets are shown in US published application 2013/0089710, US published application US 2013/0052429, US published application 2013/0017361, and U.S. Pat. No. 7,614,186. The entirety of these patents and patent applications are incorporated herein by reference.

Multiwall sheets exhibiting both low weight and high load carrying capacity can drive increased acceptance and use of multiwall sheets in building and industrial applications. Disclosed herein are multiwall sheets having minimal internal members. These sheets can provide exceptional flexural rigidity while minimizing the size and number of internal members thereby minimizing sheet weight. The design of these sheets optimizes the placement of internal members to further reduce weight while retaining high stiffness.

To achieve low weight, multiwall sheets were developed with minimum number of internal parts. Ribs, located between the outer walls of the multiwall sheet, reinforce the walls to resist deflection under an applied load (i.e. provide increased sheet flexural stiffness). Additional mid-ribs, also called an internal mid skin, extend between the ribs to provide support to the ribs by simultaneously providing additional orthotropic stiffness and resistance to nonlinear mid plane stretching (i.e. stretching along the natural axis of the sheet). The beneficial effects of the mid-ribs can be further increased by slanting the mid-rib non-parallel to the wall, and connecting the mid-rib directly to adjacent ribs, where the mid rib is free of attachment to the outside walls of the multiwall sheet.

The mid-ribs can divide the internal structure between adjacent ribs of the sheet into two cells (versus a single cell when only ribs, and no mid-ribs, are present). In addition to providing improved flexural structural rigidity the mid-ribs can also add a thermal break (e.g., barrier) between the outer walls of the multiwall sheet, which can increase the resistance to energy being transferred perpendicularly through the wall. The mid-rib can offer an improved thermally insulative characteristic to the multiwall sheet in comparison to sheets without a mid-rib (e.g., particularly in the case of a low conductance plastic multiwall sheets). Furthermore, by slanting the mid-rib and attaching the mid-rib only to adjacent ribs and not to the outer walls of the sheet, the thermal insulation property of the sheet can be improved relative to multiwall sheets where mid-ribs attach to the outer walls. The characteristics (e.g. thermal, acoustic, or optical characteristics) of the multiwall sheet can be further manipulated through the use of fillers or surface treatments.

The multiwall sheet can be formed from a plastic material, such as thermoplastic resins, thermosets, and combinations comprising at least one of the foregoing. The multiwall sheet walls, ribs and mid-ribs can be formed from the same plastic material or can be formed from different plastic materials (e.g., plastics having a different chemical formula, chemical composition, or the like). Plastic material can be chosen which have similar coefficients of thermal expansion to reduce the occurrence of damage due to differences in thermal expansion between the walls, ribs, and/or mid-ribs.

Possible thermoplastic resins that may be employed to form the multiwall sheet walls, ribs, and mid-ribs include, but are not limited to, oligomers, polymers, ionomers, dendrimers, copolymers such as graft copolymers, block copolymers (e.g., star block copolymers, random copolymers, and the like) and combinations comprising at least one of the foregoing. Examples of such thermoplastic resins include, but are not limited to, polycarbonates (e.g., blends of polycarbonate (such as, polycarbonate-polybutadiene blends, copolyester polycarbonates)), polystyrenes (e.g., copolymers of polycarbonate and styrene, polyphenylene ether-polystyrene blends), polyimides (e.g., polyetherimides), acrylonitrile-styrene-butadiene (ABS), polyalkylmethacrylates (e.g., polymethylmethacrylates (PMMA)), polyesters (e.g., copolyesters, polythioesters), polyolefins (e.g., polypropylenes (PP) and polyethylenes, high density polyethylenes (HDPE), low density polyethylenes (LDPE), linear low density polyethylenes (LLDPE)), polyamides (e.g., polyamideimides), polyarylates, polysulfones (e.g., polyarylsulfones, polysulfonamides), polyphenylene sulfides, polytetrafluoroethylenes, polyethers (e.g., polyether ketones (PEK), polyether etherketones (PEEK), polyethersulfones (PES)), polyacrylics, polyacetals, polybenzoxazoles (e.g., polybenzothiazinophenothiazines, polybenzothiazoles), polyoxadiazoles, polypyrazinoquinoxalines, polypyromellitimides, polyquinoxalines, polybenzimidazoles, polyoxindoles, polyoxoisoindolines (e.g., polydioxoisoindolines), polytriazines, polypyridazines, polypiperazines, polypyridines, polypiperidines, polytriazoles, polypyrazoles, polypyrrolidones, polycarboranes, polyoxabicyclononanes, polydibenzofurans, polyphthalamide, polyacetals, polyanhydrides, polyvinyls (e.g., polyvinyl ethers, polyvinyl thioethers, polyvinyl alcohols, polyvinyl ketones, polyvinyl halides, polyvinyl nitriles, polyvinyl esters, polyvinylchlorides), polysulfonates, polysulfides, polyureas, polyphosphazenes, polysilazanes, polysiloxanes, fluoropolymers (e.g., polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), fluorinated ethylene-propylene (FEP), polyethylene tetrafluoroethylene (ETFE)) and combinations comprising at least one of the foregoing.

More particularly, the thermoplastic resin used in the multiwall sheet walls, ribs, and mid-ribs can include, but is not limited to, polycarbonate resins (e.g., Lexan™ resins, commercially available from SABIC Innovative Plastics), polyphenylene ether-polystyrene resins (e.g., Noryl™ resins, commercially available from SABIC Innovative Plastics), polyetherimide resins (e.g., Ultem™ resins, commercially available from SABIC Innovative Plastics), polybutylene terephthalate-polycarbonate resins (e.g., Xenoy™ resins, commercially available from SABIC Innovative Plastics), copolyestercarbonate resins (e.g. Lexan™ SLX resins, commercially available from SABIC Innovative Plastics), and combinations comprising at least one of the foregoing resins. Even more particularly, the thermoplastic resins can include, but are not limited to, homopolymers and copolymers of a polycarbonate, a polyester, a polyacrylate, a polyamide, a polyetherimide, a polyphenylene ether, or a combination comprising at least one of the foregoing resins. The polycarbonate can comprise copolymers of polycarbonate (e.g., polycarbonate-polysiloxane, such as polycarbonate-polysiloxane block copolymer), linear polycarbonate, branched polycarbonate, end-capped polycarbonate (e.g., nitrile end-capped polycarbonate), and combinations comprising at least one of the foregoing, for example, a combination of branched and linear polycarbonate.

The multiwall sheet walls, ribs, and mid-ribs can include various additives ordinarily incorporated into polymer compositions of this type, with the proviso that the additive(s) are selected so as to not significantly adversely affect the desired properties of the sheet, in particular, transparency, deflection, stress, and flexural stiffness. Such additives can be mixed at a suitable time during the mixing of the components for forming the multiwall sheet. Exemplary additives include impact modifiers, fillers, reinforcing agents, antioxidants, heat stabilizers, light stabilizers, ultraviolet (UV) light stabilizers, plasticizers, lubricants, mold release agents, antistatic agents, colorants (such as carbon black and organic dyes), surface effect additives, radiation stabilizers (e.g., infrared absorbing), flame retardants, and anti-drip agents. A combination of additives can be used, for example a combination of a heat stabilizer, mold release agent, and ultraviolet light stabilizer. In general, the additives are used in the amounts generally known to be effective. The total amount of additives (other than any impact modifier, filler, or reinforcing agents) is generally 0.001 wt % to 5 wt %, based on the total weight of the composition of the multiwall sheet.

In addition to flexural stiffness, deflection, and lower edge stress, the polymeric material can be chosen to exhibit sufficient impact resistance such that the sheet is capable of resisting breakage (e.g., cracking, fracture, and the like) caused by impact (e.g., hail, birds, stones, and so forth). Therefore, polymers exhibiting an impact strength greater than or equal to about 7.5 foot-pounds per square inch, ft-lb/int (4.00 Joules per square centimeter, J/cm²), or more specifically, greater than about 10.0 ft-lb/int (5.34 J/cm²) or even more specifically, greater than or equal to about 12.5 ft-lb/int (6.67 J/cm²) are desirable, as tested per ASTM D-256-93 (Izod Notched Impact Test). Further, desirably, the polymer has ample stiffness to allow for the production of a sheet that can be employed in applications wherein the sheet is generally supported and/or clamped on two or more edges of the sheet (e.g., clamped on all four edges), such as in greenhouse applications comprising tubular steel frame construction. Sufficient stiffness herein is defined as polymers comprising a Young's modulus (e.g., modulus of elasticity) that is greater than or equal to about 1×10⁹ N/m², or, more specifically, 1×10⁹ to 20×10⁹ N/m², or, still more specifically, 2×10⁹ to 10×10⁹ N/m².

The multiwall sheet can be transparent, depending upon the desired end use. For example, multiwall sheet can have a transparency of greater than or equal to 80%, specifically, greater than or equal to 85%, or, more specifically, greater than or equal to 90%, or, even more specifically, greater than or equal to 95%, or, still more specifically, greater than or equal to 99%. Transparency is described by two parameters, percent transmission and percent haze. Percent transmission and percent haze for laboratory scale samples can be determined using ASTM D1003-00, procedure B using CIE standard illuminant C. ASTM D-1003-00 (Procedure B, Spectrophotometer, using illuminant C with diffuse illumination with unidirectional viewing) defines transmittance as:

$\begin{matrix} {{\% T} = {\left( \frac{I}{I_{O}} \right) \times 100\%}} & \lbrack 1\rbrack \end{matrix}$

wherein: I=intensity of the light passing through the test sample

-   -   I_(o)=Intensity of incident light.

A multiwall sheet can be formed from various polymer processing methods, such as extrusion or injection molding, if produced as a unitary structure. Continuous production methods, such as extrusion, generally offer improved operating efficiencies and greater production rates than non-continuous operations, such as injection molding. Specifically, a single screw extruder can be employed to extrude a polymer melt (e.g., polycarbonate, such as Lexan*, commercially available from SABIC Innovative Plastics). The polymer melt can be fed to a profile die capable of forming an extrudate having the cross-section of the multiwall sheet 30 illustrated in any of the FIGS. The multiwall sheet 30 travels through a sizing apparatus (e.g., vacuum bath comprising sizing dies) and is then cooled below its glass transition temperature (e.g., for polycarbonate, about 297° F. (147° C.)).

After the panel has cooled, it can be cut to the desired length utilizing, for example, an extrusion cutter such as an indexing in-line saw. Once cut, the multiwall sheet can be subjected to secondary operations before packaging. Exemplary secondary operations can include coating, embossing, replication, annealing, printing, attachment of fastening members, trimming, further assembly operations, and/or any other desirable processes. The size of the extruder, as measured by the diameter of the extruder's screw, is based upon the production rate desired and calculated from the volumetric production rate of the extruder and the cross-sectional area of the panel. The cooling apparatus can be sized (e.g., length) to remove heat from the extrudate in an expeditious manner without imparting haze.

Haze can be imparted when a polymer (e.g., polycarbonate) is cooled rapidly. Therefore, the cooling apparatus can operate at warmer temperatures (e.g., greater than or equal to about 100° F. (39° C.), or more specifically, greater than or equal to 125° F. (52° C.), rather than colder temperatures (e.g., less than 100° F. (39° C.), or more specifically, less than or equal to about 75° F. (24° C.)) to reduce hazing. If warmer temperatures are employed, the bath length can be increased to allow ample time to reduce the extrudate's temperature below its glass transition temperature. The size of the extruder, cooling capacity of the cooling apparatus, and cutting operation can be capable of producing the multiwall sheet at a rate of greater than or equal to about 5 feet per minute. However, production rates of greater than about 10 feet per minute, or even greater than about 15 feet per minute can be achieved if such rates are capable of producing surface features that comprise the desired attributes.

Coextrusion methods can also be employed for the production of the multiwall sheet. Coextrusion can be employed to supply different polymers to any portion of the multiwall sheet's geometry to improve and/or alter the performance of the sheet and/or to reduce raw material costs. One skilled in the art would readily understand the versatility of the process and the myriad of applications in which coextrusion can be employed in the production of multiwall sheets.

A more complete understanding of the components, processes, and apparatuses disclosed herein can be obtained by reference to the accompanying drawings. These figures (also referred to herein as “FIG.”) are merely schematic representations based on convenience and the ease of demonstrating the present disclosure, and are, therefore, not intended to indicate relative size and dimensions of the devices or components thereof and/or to define or limit the scope of the exemplary embodiments. Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.

FIG. 1-3 illustrate a multiwall sheet 30 comprising walls, where the walls include a first wall 2 with a thickness T₂, a second wall 4 with a thickness T₄, a plurality of ribs 8, with a thickness T₈, extending from the inside surface 41 of the first wall 2 to the inside surface 42 of the second wall 4, and a plurality of mid-ribs 6 with a thickness T₆ extending between adjacent ribs (i.e. where one mid-rib 6 extends between each two adjacent ribs 8). The first wall 2 and the second wall 4 can be the outermost walls of the multiwall sheet 30, each having an outer surface 40, 43 opposite an inner surface 41, 42 from which the ribs 8 extend. The first wall 2 can be parallel to the second wall 4, or can be substantially parallel to the second wall 4 throughout the multiwall sheet 30 (e.g., not perfectly parallel throughout the l-w plane in the accompanying figures, but also not intersecting, accommodating slight variations in the orientation during processing). The overall sheet thickness T can be uniform or substantially uniform throughout the multiwall sheet 30 in the directions of both the l-axis and w-axis (e.g., not perfectly uniform throughout the l-w plane in the accompanying figures, accommodating for slight variations in the orientation during processing).

The multiwall sheet width W can be measured along the w-axis dimension. The multiwall sheet length L can be measured along the l-axis dimension. The multiwall sheet thickness T can be measured along the t-axis dimension. The sheet thickness T can be measured from the outside surface 40 of the first wall 2 to the outside surface 43 of the second wall 4. The ribs 8 of the multiwall sheet 30 can extend between the first and second walls and can be perpendicular to the first and second wall or, the ribs 8 can be substantially perpendicular to the first and second walls (e.g. not perfectly perpendicular to the first and second walls across the width W of the multiwall sheet 30, accommodating for slight variations in the orientation during processing). The distance between ribs, or rib spacing, L_(o) can be uniform (e.g. equal throughout the length of the multiwall sheet, or substantially equal accommodating slight variations in rib thicknesses resulting from processing) or, the rib spacing L_(o) can be non-uniform.

FIG. 1 illustrates a cross-sectional view, taken along a t-w plane, of a section of the multiwall sheet 30. Each mid-rib 6 of the plurality of mid-ribs has a mid-rib angle 18 which defines the trajectory of a mid-rib between adjacent ribs 8. The mid-rib angle can be equal to 90° or, a mid-rib angle 18 can be substantially equal to 90° (e.g. not exactly 90°, accommodating slight variations resulting from processing). Also illustrated, are a first initial offset distance 16, measured from the first wall 2 to the left hand side of a mid-rib 6, and a second initial offset distance 14, measured from the second wall 4 to the left hand side of a mid-rib 6. These initial offset distances (14, 16) can be equal to one another or, these initial offset distances can be substantially equal to one another (e.g. not exactly equal, accommodating for slight variations resulting from processing). Similarly, a first terminal offset distance 20, measured from the first wall 2 to the right hand side of a mid-rib 6, and a second terminal offset distance 22, measured from the second wall 4 to the right hand side of the mid-rib 6 are shown. These terminal offset distances (20, 22) can be equal to one another or, these terminal offset distances can be substantially equal to one another (e.g. not exactly equal, accommodating for slight variations resulting from processing). All four offset distances (i.e. first initial offset distance 16, second initial offset distance 14, first terminal offset distance 20, and second terminal offset distance 22) can be equal to one another, or all four offset distances can be substantially equal to one another (accommodating for slight variations resulting from processing). The plane of the mid-rib 6 can bisect the distance between the plane of the first wall 2 and plane of the second wall 4. The plane of the mid-rib 6 can be offset from a plane that bisects the distance between the plane of the first wall 2 and a plane of the second wall 4, such that the mid-rib 6 is shifted closer to the first wall 2 or closer to the second wall 4.

FIG. 2 illustrates a cross-sectional view, taken along a t-w plane, of a cross section of a multiwall sheet 30 similar to that of FIG. 1, but with a plurality of inclined mid-ribs 36 extending between ribs 8. This mid-rib 6 configuration can be referred to as an “S” configuration. In this case, the inclined mid-ribs 36 form a repeating pattern, much like a sawtooth wave pattern. The inclined mid-ribs 36 are oriented such that the mid-rib angles 18 are less than 90° relative the ribs 8 with which they attach on the left hand side of each mid-rib 6. The plane in which each inclined mid-rib 36 lies can be parallel (as shown in FIG. 2), or can be nonparallel (not shown). Each of the mid-rib angle 18, first initial offset distance 16, first terminal offset distance 20, second initial offset distance 14, second terminal offset distance 22, or a combination including at least one of the foregoing, of each inclined mid-rib 36 can be different, the same, random, form a pattern, or a combination including at least one of the foregoing along the length of the multiwall sheet 30.

The total sheet thickness T of the multiwall sheet as measured in the t-axis dimension (see FIGS. 1-3) can be 4 millimeters (mm) to 100 mm, for example, 8 mm to 55 mm, or, 10 mm to 32 mm, but generally greater than or equal to 6 mm. The multiwall sheet can have a thickness of 10 mm, or, more specifically, 12 mm, or, more specifically, 16 mm, or, still more specifically, 20 mm, or, yet still more specifically, 22 mm.

The first and second wall thicknesses, T₂ and T₄, as measured along the t-axis direction (see FIGS. 1-3) can be 0.1 mm to 10 mm, for example, 0.2 mm to 1.2 mm, or, 0.3 mm to 1.0 mm. The multiwall sheet can have a wall thickness of 0.6 mm, or, more specifically, 0.8 mm, or, still more specifically, 1 mm, or, yet still more specifically, 1.2 mm.

The mid-rib thickness T₆ as measured in the direction perpendicular to the plane of a given mid-rib (see FIGS. 1-3) can be 0.02 mm to 2 mm, for example, 0.1 mm to 0.5 mm, or, 0.15 mm to 0.25 mm. The multiwall sheet can have a mid-rib thickness of 0.05 mm, or, more specifically, 0.1 mm, or, still more specifically, 0.25 mm, or, yet still more specifically, 0.5 mm.

The rib thickness T₈ as measured along the l-axis direction (see FIGS. 1-3) can be 0.02 mm to 2 mm, for example, 0.2 mm to 0.8 mm, or, 0.4 mm to 0.5 mm The multiwall sheet can have a rib thickness of 0.25 mm, or, more specifically, 0.5 mm, or, still more specifically, 0.75 mm, or, yet still more specifically, 1 mm.

The first initial offset distance 16 of the mid-rib 6 can be 1%-99% of the multiwall total sheet thickness T, for example 5% to 95%, or, 10% to 90%.

The first terminal offset distance 20 of the mid-rib 6 can be 1%-99% of the multiwall total sheet thickness T, for example 5% to 95%, or, 10% to 90%.

The second initial offset distance 14 of the mid-rib 6 can be 1%-99% of the multiwall total sheet thickness T, for example 5% to 95%, or, 10% to 90%.

The second terminal offset distance 22 of the mid-rib 6 can be 1%-99% of the multiwall total sheet thickness T, for example 5% to 95%, or, 10% to 90%.

The Applicants found that when the mid-rib 6 was attached at the point where the first wall 2 (or second wall 4) and a rib 8 connected (the corner formed between a wall and a rib) that material would accumulate in the corners, due to practical limitations of forming the sheet. The accumulation of material increased the sheet weight without improving the structural, thermal, acoustic, or optical performance of the sheet. By moving the mid-rib 6 attachment point away from the corner, offsetting the mid-rib 6 from the wall, the sheet could be formed without excess material and the sheet properties would be maximized for a given unit weight (also called specific weight, specific sheet weight, or weight per sheet area).

FIG. 3 illustrates a cross-sectional view, taken along a t-w plane, of a section of a multiwall sheet 30 having an alternating pattern of inclined mid-ribs 36 and declined mid-ribs 46. This illustrates an alternating mid-rib pattern wherein a declined mid-rib 46 (where the mid-rib angle 18 is greater than 90°) can be adjacent to an inclined mid-rib 36, much like a triangular wave pattern. In FIG. 3 the alternating pattern is defined by a one to one alternation between declined mid-ribs 46 and inclined mid-ribs 36. The pattern is repeated along the length L of the multiwall sheet 30, as measured along the l-axis, such that the mid-ribs sandwiching each rib are mirror images of one another—the mid-ribs are symmetric about a given rib (only ribs attached to an edge 10 or 12 have no corresponding mid-rib mirroring them). The mid-rib pattern can also comprise alternating, non-symmetric mid-ribs (not shown) wherein the mid-rib angles can vary, rib spacing L_(o) can be unequal between adjacent ribs, and/or the offset distances associated with each mid-rib can vary throughout the sheet.

When the mid-ribs 6 are oriented as declined mid-ribs 46 the plane in which each declined mid-rib 46 lies can be parallel, or can be nonparallel. The mid-rib angle 18 of each declined mid-rib 46 can be equal along the length of the multiwall sheet 30 measured along the l-axis or, the mid-rib angle of declined each mid-rib can be unequal along the length of the sheet measured along the l-axis.

The mid-rib angle 18 can define the orientation of the mid-rib relative to an adjacent rib. For the sake of clarity the mid-rib angle 18 has been measured relative to the left hand side rib 8 to which the mid-rib attaches. This mid-rib angle 18 can be any angle greater than 0° and less than 180° (relative to the rib 8 from which the mid-rib angle 18 is measured). The mid-ribs 6 can extend between the ribs 8 wherein the mid-ribs 6 do not attach to either outer wall of the multiwall sheet (i.e. the mid-rib does not directly attach to either outer wall 2 or 4). Still more specifically, the mid-ribs can be inclined 36 or declined 46 (as shown in FIG. 3), wherein the angle of the mid-rib relative to the rib 8 from which it extends does not equal 90°. In other words, the mid-ribs can be non-perpendicular to the ribs to which they attach. More specifically, the inclined mid-rib 36 can have a mid-rib angle 18 of less than 90°, or, 5° to 75°, or, 25° to 35°, or, 30°. Additionally, the declined mid-rib 46 can have a mid-rib angle 18 of greater than 90°, or, 105° to 175°, or, 145° to 155°, or, 150°. Furthermore, the mid-rib angle of each mid-rib can vary from the mid-rib angle of another mid-rib (adjacent or, subsequent, non-adjacent mid-rib) along the length of the multiwall sheet.

Additionally, the rib spacing L_(o) can be greater than, equal to, or less than the gauge of the multiwall sheet (i.e. multiwall sheet thickness T) and can vary along the length L of the sheet. Specifically, the rib spacing L_(o) can be 25%-400% of the multiwall sheet thickness T, for example, 50% to 200%, or, 75% to 150%, or, 95% to 105%, or, 100%.

The rib spacing L_(o) can be the length of a mid-rib 6 projected onto l-axis and measured along the l-axis, (e.g., the horizontal length of the mid-rib). The mid-rib height 26 (MRH) can be the length of a mid-rib 6 projected onto the t-axis and measure along the t-axis, (e.g., the vertical length of the mid-rib). The dimensional parameters of the mid-ribs 6 can also be defined in terms of a mid-rib angle 18, a rib spacing L_(o), and a mid-rib height 26 (MRH) or any combination comprising two of the foregoing parameters by using basic trigonometric functions. For example, the mid-rib height 26 can be determined from the rib spacing L_(o) and the mid-rib angle 18, or the rib spacing L_(o) can be determined from the mid-rib height 26 and the mid-rib angle 18, or the mid-rib angle 18 can be determined from the mid-rib height 26 and the rib spacing L_(o) by using the tangent relationship between the three parameters when the rib 8 is perpendicular to the first wall 2:

$\begin{matrix} {{\tan (\theta)} = \frac{L_{O}}{MRH}} & \lbrack 2\rbrack \end{matrix}$

Where, θ=the mid-rib angle 18 for inclined mid-ribs 36, or

-   -   θ=the mid-rib angle 18 minus 90° for declined mid-ribs 46,     -   L_(o)=the rib spacing, and     -   MRH=mid-rib height 26.         In the case of the declined mid-rib 46 it is noted that the         mid-rib angle 18 will be greater than 90°. To use the         relationship of Equation 2 for a declined mid-rib 46, the         mid-rib angle 18 will need to have 90° subtracted from it, this         will convert the obtuse triangle described by a declined mid-rib         46 to a right triangle, with sides equal to the mid-rib height         26 and rib spacing L_(o). Other trigonometric functions can be         used to fully define a particular mid-rib 6 if given other         parameters. For example, if given the mid-rib length 7 and         mid-rib height 26 then cosine can be used. Furthermore, in the         case where the rib 8 is not perpendicular to the first wall 2         then other trigonometric identities can be employed to fully         define the dimensions of the mid-rib 6 (e.g. the laws of sines,         cosines, and tangents).

A repeating mid-rib pattern as used herein can refer to a pattern comprising adjacent inclined mid-ribs 36, or adjacent declined mid-ribs 46. An alternating mid-rib pattern as used herein can refer to a pattern comprising at least one inclined mid-rib 36 adjacent to at least one declined mid-rib 46. A multiwall sheet 30 can be comprised of a repeating mid-rib pattern, an alternating mid-rib pattern, or a combination comprising at least one of the foregoing patterns. Furthermore, the initial and terminal offset distances of each mid-rib can vary from mid-rib 6 to mid-rib 6 (e.g., the initial and terminal offset distances of adjacent mid-ribs can be different).

The disclosed structural parameters (e.g., wall thicknesses, rib thicknesses, mid-rib thicknesses, mid-rib angles, overall sheet thickness, offset distances, mid-rib patterns (e.g., alternating, repeating or combinations comprising at least one of the foregoing), or a combination including at least one of the foregoing) can be adjusted to achieve a desired multiwall sheet performance and/or physical characteristic (e.g. stiffness, deflection, heat transfer, optical characteristic, acoustic characteristic, weight, dimension, and the like). These adjustments can be made locally (e.g., within selected areas of a multiwall sheet 30) and/or throughout the entire multiwall sheet 30 to attain the desired multiwall sheet performance.

FIG. 4 illustrates the load versus deflection characteristics of the multiwall sheet 30 of FIG. 1-2, for a given span, engagement distance (e.g., support length), support condition and loading. Computational results of deflection versus wind pressure for the multiwall sheet of FIG. 1 are shown in curve 120. Computational results of deflection versus wind pressure for the multiwall sheet 30 of FIG. 2 are shown in curve 100. Experimental results of deflection versus wind pressure for the multiwall sheet of FIG. 1 are shown in curve 140. From FIG. 4 it can be seen that the multiwall sheet 30 of FIG. 2, comprising inclined mid-ribs in a repeating pattern, show improved performance in comparison to the multiwall sheet 30 of FIG. 1 which comprises mid-ribs which are substantially parallel to the outer walls. Specifically, the deflection of the multiwall sheet of FIG. 2 at a wind pressure of 1000 N/m² is nearly 50% less than that of the multiwall sheet of FIG. 1 under the same conditions. Specifically, the multiwall sheet 30 of FIG. 2, comprising inclined mid-ribs in a repeating pattern, exhibited a deflection at a wind pressure of 1000 N/m² of less than or equal to 70 mm, or, less than or equal to 60 mm, or, less than or equal to 51 mm, or, less than or equal to 45 mm, or, less than or equal to 40 mm, or less than or equal to 38 mm.

FIG. 5 illustrates deflection versus multiwall sheet unit weight (sheet weight per sheet area) for the samples of multiwall sheets of FIG. 1, FIG. 2, and FIG. 3. The multiwall sheets were each rectangular having a length of 980 mm and a width of greater than two times the length. The rectangular sheets were clamped continuously along all four edges with an engagement distance of 24 mm (i.e., the distance inboard of the sheet wherein the support contacts the first and second walls of the sheet), and were subjected to 1000 N/m² wind pressure. From FIG. 5 it can be seen that while a deflection of less than or equal to 51 mm was achievable with the multiwall sheets of FIG. 1 (data points 240) the weight per area of the sheet needed to achieve such deflection was significantly higher than that of multiwall sheets of FIG. 2 and FIG. 3 (data points 260). Specifically, to achieve less than or equal to 51 mm deflection at 1000 N/m² load under the described conditions, a multiwall sheet with a configuration of FIG. 1 had a unit weight of about 2.6 kilogram per square meter (kg/m²) compared to a unit weight of about 1.6 kg/m² for a sheet with the configuration of either FIG. 2 or FIG. 3.

FIGS. 6-9 illustrate cross-sectional views, taken along a t-y plane of multiwall sheets having an “X”, “V”, “N”, and “S” mid-rib configuration. A multiwall sheet 400 having an “X” mid-rib configuration is shown in FIG. 6. A multiwall sheet 410 having a “V” mid-rib configuration is shown in FIG. 7. A multiwall sheet 420 having an “N” mid-rib configuration is shown in FIG. 8. A multiwall sheet 430 having an “S” mid-rib configuration is shown in FIG. 9.

FIG. 10 illustrates computational deflection versus flexural load for multiwall sheets having the mid-rib configurations presented in FIGS. 6-9. Curve 310 corresponds to the “V” midrib configuration of FIG. 7. Curve 320 corresponds to the “N” mid-rib configuration of FIG. 8. Curve 330 corresponds to the “X” mid-fib configuration of FIG. 6. Curve 340 corresponds to the “S” mid-rib configuration of FIG. 9. The computational simulations were conducted for multiwall sheets where each sheet was 1,050 mm in length (measured on the l-axis, as previously described) and having a width greater than two times the length. The sheets had an overall sheet thickness T of 16 mm, a rib distance L_(o) of 20 mm, and a unit weight of 3.3 kg/m², the mid rib thickness T₆ for the multiwall sheet having the “X” mid-rib configuration was halved to keep the same unit weight. Each sheet was supported continuously along each of the four sides with an engagement distance of 20 mm and deflection of each sheet was simulated as a function of wind pressure. The results demonstrate that a catastrophic buckling failure, indicated by a sudden change in slope, was significantly higher for the “S” mid-rib configuration (curve 340) in comparison to the other configurations. Specifically, the buckling failure of the sheet having the “X”, “V”, “N”, and “S” mid-rib configurations were 2,300 N/m², 2,400 N/m², 2,700 N/m², and 4,500 N/m² respectively. As can be seen, the load at which buckling failure occurs for the “S” mid-rib configured sheet is approximately 96% higher than the “X” mid-rib configured sheet (curve 330), 88% higher than the “V” mid-rib configured sheet (curve 310), and 67% higher than the “N” mid-rib configured sheet (curve 320). These results also show that at lower wind loads (e.g. 1,000 N/m²) the flexural stiffness, indicated by the amount of deflection, is virtually identical.

FIG. 11 illustrates a multiwall sheet 30 as used in a naturally lit structure as a roof or wall section of a glazing system, such as for a greenhouse, solar roof collector, conservatory, stadium, atrium, foyer, sunroom, sunroof, skylight, pool enclosure, lanai, porch, veranda, and the like.

FIG. 12 illustrates a cross sectional view, taken along in the t-l plane, of a multiwall sheet 30 of length L. The multiwall sheet is engaged by engagement members 50 on two sheet edges 10, 12 to an engagement distance E equal to two times the rib spacing L_(o). The engagement distance can be determined as a function of the rib spacing, and can be equivalent to 1 to 8 times the length of the rib spacing L_(o), for example 1 to 4, or, 1 to 2 times the rib spacing. Alternatively, the engagement distance can be independent of the rib spacing and the engagement distance can be from 5 mm to 50 mm, for example, 10 mm to 30 mm, or, 20 mm.

FIG. 13 illustrates a multiwall sheet 30 having a length L, extending along the l-axis, width W, extending along the w-axis, and a total sheet thickness T, extending along the t-axis.

The multiwall sheet disclosed herein can be formed as an integral sheet through an extrusion (e.g., co-extrusion) or the components of the multiwall sheet can be formed separately and fashioned together with any well-known technique.

The multiwall sheet disclosed herein can exhibit very similar deflection under suction and pressurized conditions when the force under each condition has the same magnitude acting normal to the sheet. A “suction” condition as used herein can refer to when a side of the multiwall sheet is exposed to pressure below the pressure acting on the opposite side of the sheet. A “pressurized” condition as used herein can refer to when a side of the multiwall sheet is exposed to a pressure above the pressure acting on the opposite side of the sheet. The only difference between these two conditions is the direction of the force vector acting on the sheet and consequently the direction of displacement (if any). Under a suction condition the absolute value of the displacement can be 75% to 125% of the displacement under a pressurized condition of the same magnitude (i.e. the size of the force, or length of the force vector, not the direction in which the force is acting) for example, 90% to 110%, or, 95% to 105%, or, 99% to 101%. The multiwall sheet can provide similar performance under both suction and pressure loading conditions. Such similar performance under opposing forces can reduce or eliminate the need to mistake-proof (e.g., identifying an outer side, inner side, and the like) the sheets to avoid improper installation.

Rib spacing can relate to light transmission, or optical clarity, and stiffness. Stiffness can improve with reduced rib spacing and optical clarity can improve with increase rib spacing. Thus, rib spacing can be optimization for a given end use or multiwall sheet application.

Example 1

Computational analysis was used to predict the deflection performance of rectangular multiwall sheets which were secured continuously along all four edges of the sheet to an engagement distance of 24 mm. The analysis simulated a wind load of 1000 N/m² and the resulting maximum deflection was predicted for each sheet. The sheets were each 980 mm in length, and width was greater than two times the length. The samples tested had different total sheet, wall, rib, and mid-rib thicknesses. In addition to varying thicknesses, the mid-rib height, and slanting distance (analogous to second initial offset distance 14 in FIG. 2) were also varied between samples. The different sheet designs (thicknesses, height, and distances) resulted in different weights for each sample. The results of the computational experimental results are summarized in Table 1.

TABLE 1 Sam- De- ple flec- No. TST¹ ST² RD³ RT⁴ MD⁵ SD⁶ DT⁷ Weight tion 1 16 0.5 12 0.4 12 2 0.2 2.131 32.91 2 16 0.6 12 0.4 12 2 0.2 2.363 25.62 3 16 0.7 12 0.4 12 2 0.2 2.595 22.21 4 16 0.8 12 0.4 12 2 0.2 2.827 19.82 5 16 0.5 12 0.5 12 2 0.2 2.279 28.02 6 16 0.6 12 0.5 12 2 0.2 2.509 23.08 7 16 0.7 12 0.5 12 2 0.2 2.739 19.94 8 16 0.8 12 0.5 12 2 0.2 2.969 17.69 9 12 0.5 12 0.4 9 1.5 0.2 1.932 51.08 10 12 0.6 12 0.4 9 1.5 0.2 2.164 35.61 11 12 0.7 12 0.4 9 1.5 0.2 2.396 30.44 12 12 0.8 12 0.4 9 1.5 0.2 2.628 27.07 13 12 0.5 12 0.5 9 1.5 0.2 2.040 40.52 14 12 0.6 12 0.5 9 1.5 0.2 2.270 32.80 15 12 0.7 12 0.5 9 1.5 0.2 2.500 28.30 16 12 0.8 12 0.5 9 1.5 0.2 2.730 25.14 17 10 0.3 12 0.4 7.5 1.25 0.15 1.302 76.54 18 10 0.4 12 0.4 7.5 1.25 0.15 1.534 48.56 19 10 0.5 12 0.4 7.5 1.25 0.15 1.766 40.75 20 10 0.6 12 0.4 7.5 1.25 0.15 1.998 36.30 21 10 0.3 12 0.5 7.5 1.25 0.15 1.395 56.04 22 10 0.4 12 0.5 7.5 1.25 0.15 1.625 44.59 23 10 0.5 12 0.5 7.5 1.25 0.15 1.855 38.52 24 10 0.6 12 0.5 7.5 1.25 0.15 2.085 34.44 Notes: All dimensions are in mm ¹TST = Total Sheet Thickness (analogous to gauge or sheet thickness T) ²ST = Skin Thickness (analogous to wall thickness T₂, T₄) ³RD = Rib Distance (analogous to rib spacing L_(o)) ⁴RT = Rib Thickness ⁵MD = Mid-rib Distance (analogous to mid-rib height 26) ⁶SD = Slanting Distance (analogous to second initial offset distance 14 in FIG. 2) ⁷DT = Mid-rib Thickness

Example 2

Computational nonlinear structural analysis was performed on multiwall sheets of FIG. 1 and FIG. 2. The sheets were simulated to have all four edges clamped with an engagement of 20 mm, and were subjected to 1000 N/m² load. The unit weight and thickness of the sheets of FIG. 1 and FIG. 2 was maintained at 2.3 kg/m² and 16 mm, respectively. Under the simulated conditions a maximum flexural deflection of 72.4 mm, and 41.5 mm was predicted for the multiwall sheets of the configurations shown in FIG. 1 and FIG. 2, respectively. Thus, for a given weight, load, span, engagement and support condition the multiwall sheet of the configuration shown in FIG. 2 was simulated to perform at a deflection of 57% of the multiwall sheet of the configuration shown in FIG. 1 (e.g., a 43% flexural stiffness improvement).

Example 3

Computational nonlinear structural analysis was performed on multiwall sheets of the configuration shown in FIG. 2 for both suction and pressurization (i.e. when the load was applied on each side of the multiwall sheet separately). When the sheet was simulated to be under a suction load the maximum flexural deflection was predicted to be 41.47 mm. When the sheet was simulated to be pressurized the maximum flexural deflection was predicted to be 41.53 mm. These results show that the performance for a multiwall sheet of the configuration shown in FIG. 2 was predicted to be nearly identical under either a suction load or a pressurized load implying that the sheet can have isotropic stiffness properties in directions perpendicular to the surface of the sheet. In contrast, the performance of a multiwall sheet having an “N” type mid-rib configuration (as shown in FIG. 8), can be non-identical for pressure and suction loading. Isotropic stiffness is advantageous in that performance is not sensitive to sheet orientation for pressure and suction loading. Specialized fasteners, connectors, collectors, clamps and the like can be avoided. This can also reduce or eliminate the need to mistake-proof the panels for installation personnel and/or consumers. With panels having nearly identical performance under these opposing conditions, there is no longer a need to orient the panels in a particular direction to achieve the desired stiffness performance.

Set forth below are examples of the multiwall sheets described herein, articles comprising the same, and methods of making the same.

Embodiment 1

A multiwall sheet comprising: a first wall; a second wall; a plurality of ribs extended between the first and second walls; and a plurality of mid-ribs, wherein one mid-rib extends between each two adjacent ribs, wherein the mid-ribs extend at a mid-rib angle relative to an adjoining rib, and wherein the mid-rib angle is not 90° relative to a rib to which it attaches, and wherein the mid-ribs attach only to adjacent ribs.

Embodiment 2

The multiwall sheet of Embodiment 1, wherein an initial end of a mid-rib is attached to a first rib at a first initial offset distance measured from a inside surface of the first wall.

Embodiment 3

The multiwall sheet of Embodiment 2, wherein the first initial offset distance is 10% to 90% of a total sheet thickness.

Embodiment 4

A multiwall sheet of and of Embodiments 1-3, wherein adjacent mid-ribs form a mid-rib pattern comprising a repeating mid-rib pattern, an alternating mid-rib pattern, or a combination comprising at least one of the foregoing patterns.

Embodiment 5

A multiwall sheet of any of Embodiments 1-4, wherein the sheet has a total sheet thickness of 4 mm to 100 mm.

Embodiment 6

A multiwall sheet of any of Embodiments 1-5, wherein the first wall has a thickness of 0.1 mm to 10 mm.

Embodiment 7

A multiwall sheet of any of Embodiments 1-6, wherein the second wall has a thickness of 0.1 mm to 10 mm.

Embodiment 8

A multiwall sheet of any of Embodiments 1-7, wherein the plurality of ribs each has a thickness of 0.1 mm to 10 mm.

Embodiment 9

A multiwall sheet of any of Embodiments 1-8, wherein the plurality of mid-ribs each has a thickness of 0.02 mm to 2 mm.

Embodiment 10

A multiwall sheet of any of Embodiments 1-9, wherein the sheet comprises a plastic material.

Embodiment 11

A multiwall sheet of any of Embodiments 1-9, wherein the sheet comprises a plastic material, and wherein the plastic material comprises homopolymers and copolymers of a polycarbonate, a polyester, a polyacrylate, a polyamide, a polyetherimide, a polyphenylene ether, or a combination comprising at least one of the foregoing.

Embodiment 12

A multiwall sheet of any of Embodiments 1-11, wherein a maximum sheet deflection is less than or equal to 70 mm at a wind pressure of 1000 N/m².

Embodiment 13

A multiwall sheet of any of Embodiments 1-12, wherein a maximum sheet deflection is less than or equal to 51 mm at a wind pressure of 1000 N/m².

Embodiment 14

A multiwall sheet of any of Embodiments 1-13, wherein a maximum sheet deflection is less than or equal to 38 mm at a wind pressure of 1000 N/m².

Embodiment 15

A multiwall sheet of any of Embodiments 1-14, wherein a sheet deflection under a suction condition is 75% to 125% of the sheet deflection under a pressurized condition of the same magnitude as the suction condition.

Embodiment 16

A multiwall sheet of any of Embodiments 1-15, wherein a rib spacing is 10% to 1000% of a total sheet thickness.

Embodiment 17

A multiwall sheet of any of Embodiments 1-15, wherein a rib spacing is 75% to 125% of a total sheet thickness.

Embodiment 18

A multiwall sheet of any of Embodiments 1-15, wherein a rib spacing is equal to a total sheet thickness.

Embodiment 19

A multiwall sheet of any of Embodiments 1-18, wherein the mid-rib angle is 5° to 85°, preferably 10° to 70°, preferably 20° to 45°.

Embodiment 20

A multiwall sheet of any of Embodiments 1-19, wherein the mid-rib angle is 95° to 175°, preferably 100° to 160°, preferably 110° to 135°.

Embodiment 21

A multiwall sheet of any of Embodiments 1-20, wherein the mid-ribs are between the first wall and the second wall.

Embodiment 22

A multiwall sheet of any of Embodiments 1-21, wherein the first wall and the second wall are free of attachments to the mid-ribs.

Embodiment 23

A multiwall sheet of any of Embodiments 1-22, wherein the mid-ribs do not attach to either the first or second walls.

Embodiment 24

An article comprising the multiwall sheet of any of Embodiments 1-23.

Embodiment 25

A method of making a multiwall sheet, comprising: forming a multiwall sheet; wherein the multiwall sheet comprises a first wall; a second wall; a plurality of ribs extended between the first and second walls; and a plurality of mid-ribs, wherein one mid-rib extends between each two adjacent ribs, wherein the mid-ribs extend at a mid-rib angle relative to an adjoining rib, and wherein the mid-rib angle is not 90° relative to a rib to which it attaches, wherein the mid-ribs attach only to adjacent ribs, and wherein the mid-ribs do not attach to either the first or second walls.

Embodiment 26

The method of Embodiment 25, wherein forming a multiwall sheet comprises extruding, coextruding, injection molding or a combination comprising at least one of the foregoing.

Embodiment 27

The method of any of Embodiments 25-26, further comprising cutting the multiwall sheet.

Embodiment 28

The method of any of Embodiments 25-27, further comprising processing the multiwall sheet in a secondary operation.

In general, the invention may alternately comprise, consist of, or consist essentially of, any appropriate components herein disclosed. The invention may additionally, or alternatively, be formulated so as to be devoid, or substantially free, of any components, materials, ingredients, adjuvants or species used in the prior art compositions or that are otherwise not necessary to the achievement of the function and/or objectives of the present invention.

All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other (e.g., ranges of “up to 25 wt. %, or, more specifically, 5 wt. % to 20 wt. %”, is inclusive of the endpoints and all intermediate values of the ranges of “5 wt. % to 25 wt. %,” etc.). “Combination” is inclusive of blends, mixtures, alloys, reaction products, and the like. Furthermore, the terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to denote one element from another. The terms “a” and “an” and “the” herein do not denote a limitation of quantity, and are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., the film(s) includes one or more films). Reference throughout the specification to “one embodiment”, “another embodiment”, “an embodiment”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments.

While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents. 

1. A multiwall sheet comprising: a first wall; a second wall; a plurality of ribs extended between the first and second walls; and a plurality of mid-ribs, wherein one mid-rib extends between each two adjacent ribs, wherein the mid-ribs extend at a mid-rib angle relative to an adjoining rib, and wherein the mid-rib angle is not 90° relative to a rib to which it attaches, wherein the mid-ribs attach only to adjacent ribs, and wherein the mid-ribs do not attach to either the first or second walls.
 2. The multiwall sheet of claim 1, wherein an initial end of a mid-rib is attached to a first rib at a first initial offset distance measured from a inside surface of the first wall.
 3. The multiwall sheet of claim 2, wherein the first initial offset distance is 10% to 90% of a total sheet thickness.
 4. A multiwall sheet of claim 1, wherein adjacent mid-ribs form a mid-rib pattern comprising a repeating mid-rib pattern, an alternating mid-rib pattern, or a combination comprising at least one of the foregoing patterns.
 5. A multiwall sheet of claim 1, wherein the sheet has a total sheet thickness of 4 mm to 100 mm.
 6. A multiwall sheet of claim 1, wherein the first wall has a thickness of 0.1 mm to 10 mm.
 7. A multiwall sheet of claim 1, wherein the second wall has a thickness of 0.1 mm to 10 mm.
 8. A multiwall sheet of claim 1, wherein the plurality of ribs each has a thickness of 0.1 mm to 10 mm.
 9. A multiwall sheet of claim 1, wherein the plurality of mid-ribs each has a thickness of 0.02 mm to 2 mm.
 10. A multiwall sheet of claim 1, wherein the sheet comprises a plastic material.
 11. A multiwall sheet of claim 10, wherein the plastic material comprises homopolymers and copolymers of a polycarbonate, a polyester, a polyacrylate, a polyamide, a polyetherimide, a polyphenylene ether, or a combination comprising at least one of the foregoing.
 12. A multiwall sheet of claim 1, wherein a maximum sheet deflection is less than or equal to 70 mm, at a wind pressure of 1000 N/m².
 13. A multiwall sheet of claim 1, wherein a sheet deflection under a suction condition is 75% to 125% of the sheet deflection under a pressurized condition of the same magnitude as the suction condition.
 14. A multiwall sheet of claim 1, wherein a rib spacing is 10% to 1,000% of a total sheet thickness.
 15. A multiwall sheet of claim 1, wherein a rib spacing is equal to a total sheet thickness.
 16. An article comprising the multiwall sheet of claim
 1. 17. A method of making a multiwall sheet, comprising: forming a multiwall sheet; wherein the multiwall sheet comprises a first wall; a second wall; a plurality of ribs extended between the first and second walls; and a plurality of mid-ribs, wherein one mid-rib extends between each two adjacent ribs, wherein the mid-ribs extend at a mid-rib angle relative to an adjoining rib, and wherein the mid-rib angle is not 90° relative to a rib to which it attaches, wherein the mid-ribs attach only to adjacent ribs, and wherein the mid-ribs do not attach to either the first or second walls.
 18. The method of claim 17, wherein forming a multiwall sheet comprises extruding, coextruding, injection molding or a combination comprising at least one of the foregoing.
 19. The method of claim 17, further comprising cutting the multiwall sheet.
 20. The method of claim 17, further comprising processing the multiwall sheet in a secondary operation. 